A diffusion and reaction model is presented for a gas phase membrane biofilm reactor (MBfR) that overcomes many of limitations of conventional biofilters. The model considers transfer of toluene from the gas phase through the membrane and into a toluene degrading biofilm. Data from bench-scale tests of toluene removal in an MBfR were used to validate the model. Overall mass transfer coefficients were 0.64 x 10(-3), 1.1 x 10(-3), 1.86 x 10(-3) (m s(-1)) at liquid flow rates (shell-side) of 4, 7, and 9 L min(-1), respectively. Modeled mass transfer coefficients were in good agreement with experimental values. The benchscale MBfR was capable of removing more than 98% of the influent toluene (an overall removal rate of 17 g m(-3) min(-1)) at an inlet concentration of 100 ppm(v) and a gas flow rate of 1.0 L min(-1) (gas residence time of 1.3 s in the membrane lumen). Model predictions were well in agreement with experimental values. (C) 2009 Elsevier Ltd. All rights reserved.